Micro-optical electromechanical system (MEOMS) membranes are used in a spectrum of optical applications. For example, they can be coated to be reflective and then paired with a stationary mirror to form a tunable Fabry-Perot (FP) cavity/filter. They can also be used as stand-alone reflective components to define the end of a laser cavity, for example.
Typically, membrane deflection is achieved by applying a voltage between the membrane and a fixed electrode. Electrostatic attraction moves the membrane in the direction of the fixed electrode as a function of the applied voltage. This results in changes in the reflector separation of the FP filter or cavity length in the case of a laser.
In optical systems, these membranes have advantages over cantilevered structures, for example. Membranes better maintain parallelism through the range of their deflection and tend to be more mechanically robust and have fewer relevant vibration modes.
In the past, the commercial MEOMS membranes have been produced by depositing a dielectric mirror structure over a sacrificial layer, which has been deposited on a support structure. This sacrificial layer is subsequently etched away to produce a suspended membrane structure in a release process. If a curved membrane structure is desirable, a compressive stress is cultivated in the silicon compound to induce a bow.
In number of applications, it would be desirable to fabricate membranes with predetermined optically curved surfaces that work in transmission or reflection, such as 1) refractive lens structures, including lenses with continuous curvatures or Fresnel profiles; 2) diffractive lens or mirror structures; or 3) mirror structures having continuous curvatures or Fresnel profiles.
The present invention concerns an optical membrane device and method for making such a device. This membrane is notable in that it comprises an optically curved surface. In some embodiments, this surface is optically concave and coated, for example, with a highly reflecting (HR) coating to create a curved mirror. In other embodiments, the optical surface is optically convex and coated with, preferably, an antireflective (AR) coating to function as a collimating or focusing lens.
In general, according to one aspect, the invention features an optical membrane device that comprises a support and a device layer, wherein a deflectable membrane structure is formed in the device layer. A sacrificial layer separates the support from the device layer. This sacrificial layer has been selectively removed to release the membrane structure. According to the invention, an optically curved surface has been formed on this deflectable membrane.
In one example, the curved optical surface is formed in an optical element layer that is deposited on the device layer. Alternatively, the curved optical surface is etched into the device layer.
In one implementation, the curved optical surface is an optically concave surface that has been etched into the device layer. This optically concave surface is formed as either a continuously curved, a Fresnel, or diffractive surface. In another implementation, the optical surface is an optically convex shape that is formed in a layer that has been deposited on the devices layer or that has been etched into the device layer. This optically convex shape is formed either as a continuous curved surface, a Fresnel surface or using diffractive features.
In the present implementation, the sacrificial layer defines an electrical cavity across which electrical fields are established to deflect the membrane structure in the direction of the support or stationary electrode. In one example, this membrane structure comprises a center body portion, an outer portion, and tethers that extend between the center body portion and the outer portion.
In one application, an optical coating is applied to the optically curved surface. For example, a concave mirror structure is formed by applying a multi-layer dielectric mirror coating to the curved optical surface. In another example, an anti-reflective coating is applied to a convex optical surface to thereby form a refractive or diffractive lens element.
In general, according to another aspect, the invention also features a process for fabricating an optical membrane structure. This process comprises providing a support and forming a sacrificial layer on the support. A device layer is then further formed on this sacrificial layer. A membrane structure is patterned into the device layer and the membrane structure released by the selective removal of the sacrificial layer. Finally, according to the invention, an optically curved surface is formed on the membrane structure of the device layer.
In one embodiment, the process for fabricating the curved optical surface comprises depositing a photo-resist layer and then reflowing that photo-resist layer to create a curved surface. This curved surface is then transferred into the device layer by etching the photo-resist and the device layer.
To form a convex optical surface, a columnar photo-resist layer is reflowed to form a convex surface. In contrast, to create a concave surface, a columnar blind hole is etched into a photo-resist layer or the device layer. This columnar blind hole is then over-coated with another photo-resist layer and then the resulting concave surface transferred into the device layer by etching.
In general, according to still another aspect, the invention features a process for fabricating concave mirror structures. This process comprises depositing a photo-resist layer over a well or blind hole in a substrate. In one example, this substrate is limited to simply a device layer. In another example, this substrate comprises a multi-layer structure, such as a device layer, which has been coated with another photo-resist layer. A resulting curved surface is then transferred into the substrate by etching the photo-resist and the substrate. This curved surface is then coated with a dielectric mirror coating to thereby yield a concave mirror structure.
The above and other features of the invention including various novel details of construction and combinations of parts, and other advantages, will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular method and device embodying the invention are shown by way of illustration and not as a limitation of the invention. The principles and features of this invention may be employed in various and numerous embodiments without departing from the scope of the invention.